Semiconductor devices, particularly high-speed power semiconductor devices, generate heat during use. This heat must be removed in order to prevent further heating, performance degradation and/or consequent destruction of the device. Heat dissipation is therefore key to the reliability of any given semiconductor device.
There are various known methods for dissipating heat, also referred to as heat transfer, in a semiconductor device. Heat transfer involves the movement of heat from one point to another point due to a difference in temperature between the two points. Some of the primary mechanisms by which heat can be transferred from one region to another include conduction, which is a method of heat transfer wherein heat is exchanged between two elements that are situated in close proximity to one another, convection, which refers to heat transfer in a liquid or gas by the circulation or flow of the liquid or gas from one region to another, and radiation, which refers to heat transfer through electromagnetic heat exchange in the form of waves and rays.
Conventionally, heat dissipation in a semiconductor device is often achieved by using a heat sink or similar structure affixed to the outer package of the device. Heat sinks function by extending the surface area of heat dissipating surfaces through the use of fins, usually made out of metal with good thermal conductivity such as copper or aluminum. The fabrication methods can range from extruding, bonding, folding, die-casting, forging or skiving. This approach, however, requires that there be a good thermal bonding between the semiconductor device and heat sink which are often formed of different materials. Commonly used bonding methods are die attach, thermal grease, or thermal interface materials. Good performance die attach materials such as silver paste are expensive. In addition to the cost, the bonding materials inevitably introduce multiple interfaces in the thermal path between the die and the external system that impedes heat transfer and increases thermal resistance. The thermal contact resistance caused by imperfections at the interfaces, such as, for example, micro-voids or contamination, often result in a significant increase in thermal resistance. Therefore, it is desirable to minimize the number of interfaces.
Modern electronic systems, particularly handheld devices, exert more constraints on heat dissipation of the semiconductor power devices. In these applications, the heat transfer is predominantly between power devices and the ambient air environment. However, cooling fans or conventional metal heat sinks are usually not practical for use in the system design due to battery power restrictions, among other constraints; moreover, these cooling devices cannot fit into the tight space allocated in the system. Therefore, forced convection as an effective heat transfer mechanism is not available, leaving natural convection and radiation as the only remaining viable options for heat dissipation.